The present invention relates to a method for manufacturing a drive wheel bearing device. The bearing device is for supporting a drive wheel of a vehicle such as an automobile (rear wheels of FR cars, front wheels of FF cars, and all wheels of 4WD cars), such as to be rotatable relative to a vehicle body.
There are various types of drive wheel bearing devices for automobiles in accordance with their applications. The drive wheel bearing device shown in FIG. 12, for example, is constructed with a bearing unit 1 consisting of a wheel hub 3 and a double-row bearing; an outer joint member 7a of a constant velocity joint 7 is fitted to the inner periphery of the wheel hub 3 such that torque is transmitted.
The bearing unit 1 includes double-row outer races 1a and inner races 2a, 2b, and double-row rolling elements 5 each interposed between the outer races 1a and the inner races 2a, 2b. In the illustrated example of the bearing device, one inner race 2b of the double-row inner races is formed directly on the outer periphery of the wheel hub 3, while the other inner race 2a is formed on the outer periphery of the inner ring 4 that is fitted to one end of the wheel hub 3. A nut 9 is screwed on the shaft end of the outer joint member 7a; tightening this nut 9 causes the back face 4a of the inner ring 4 to abut on the shoulder 7a1 of the outer joint member 7a for the positioning of the inner ring 4 and for the application of certain preload to the bearing.
In another construction that has been taught recently, swaging replaces tightening of the nut 9 for uniting the wheel hub 3 and the inner ring 4 and for providing preload to the bearing, so as to achieve a decrease in the size in the axial direction and weight of the bearing device. In this device, the inner ring 4 is fitted onto the wheel hub 3, and one end of the wheel hub 3 protruding from the inner ring 4 is swaged radially outward by plastic deformation as shown in FIG. 13, the positioning of the inner ring 4 and the application of preload being achieved by engagement of the swaged portion 3a and the back face 4a of the inner ring 4. The bearing device thus assembled (shown in FIG. 14) has a so-called “self-retaining” structure that enables assembly and disassembly of the bearing unit 1 and the constant velocity joint 7 while the bearing internal clearance of a preset controlled value is maintained; the structure is advantageous in that the nut 9 need not be tightened with a large torque to give preload to the bearing unit 1 but to the extent that there is no play in the mating parts of the wheel hub 3 and the outer joint member 7a. 
Incidentally, in the drive wheel of a vehicle, in general, the center of a king pin must match the joint center O′ of the constant velocity joint 7 so as to ensure good driving stability of the vehicle. However, in the construction wherein one end of the wheel hub 3 is swaged as noted above, there are inevitable variations in the axial position of the end face of the swaged portion 3a, which causes variations in the position of the joint center O′ that is determined when the end face of the swaged portion 3a is abutted on the shoulder 7a1 of the outer joint member 7a, as a result of which it is made difficult to match it with the king pin center.
Further, when forming the flange-like swaged portion 3a, there are cases where serrations 8a formed on the inner periphery of the wheel hub 3 bulge inward at the end on the side of the swaged portion 3a due to plastic material flow during the swaging, causing a decrease in the diameter as indicated by the solid line in FIG. 15. Such decreased diameter will make it necessary to apply a much larger force than usual when press-fitting the serrated shaft 8b (see FIG. 14) of the outer joint member 7a into the inner periphery of the wheel hub 3, deteriorating the operation efficiency, and in a worst case the press-fitting itself may be made impossible.
As a countermeasure of this problem, as shown in FIG. 16, the portion of the inner periphery of the wheel hub 3 before the broaching that is expected to decrease in diameter may be given a larger inside diameter Φ d2 than the diameter Φd1 of other portions to allow for the decrease, and after broaching both the small diameter part of the inside diameter Φd1 and the large diameter part of the inside diameter .Φd2 to form the serrations 8a (as indicated by the broken line), the end part 3b of the wheel hub 3 may be swaged radially outward.
With this method, however, because there are large variations in the decrease in diameter during the swaging depending on the swaging conditions, the inside diameter .Φd2 of the large diameter part must at least be closely controlled, which will raise the manufacturing cost.
Another possible measure would be to cut the portion X′ of the inner periphery of the wheel hub 3 that is expected to decrease in diameter after the swaging as shown in FIG. 17, and to form this portion as a cylindrical surface without the broaching; this would make the effective axial length of the serrations 8a shorter and could possibly lead to insufficient torque transmission to and from the constant velocity joint 7.
Prior art has shown that the thrust face of the swaged portion 3a of the wheel hub 3 that abuts on the outer joint member 7a may be formed as a flat surface by a coining or turning process (Japanese Patent Laid-Open Publication No. Hei 11-5404), but the end face of the swaged portion 3a could only be flattened to a limited degree, and even slightest undulation may create a small gap between the thrust face and the abutting shoulder 7a1 of the outer joint member 7a. The bearing takes large moment load during cornering, which may well cause resilient deformation of the bearing, whereupon the gap between the abutting faces may be widened due to the self-retaining structure that can lessen the tightening force of the nut 9, resulting in troubles such as penetration of dust or rain water through this gap. Dust or rain water penetrated into the mating parts of the wheel hub 3 and the serrated shaft 8b will cause formation of rust, which not only accelerates wear of the mating parts but causes them to stick; a large number of process steps will thereby be required for the disassembly, which is not desirable.
Another construction has been known (Japanese Patent Laid-Open Publication No. 2000-142009), in which the serrated shaft 8b of the outer joint member 7a and the wheel hub 3 are coupled together in a detachable manner using a retention ring, and in which the radial gap between the wheel hub 3 and the serrated shaft 8b is sealed by a sealing member; however, the gap between the aforementioned abutting faces is much larger than that in the nut tightening structure shown in FIG. 14 whereby a solid sealing member is required, resulting in higher costs and lower rigidity of the entire device, which is not desirable.
The present invention has been devised under these circumstances, its object being to achieve the following:
1) To prevent offsetting between the king pin center and the joint center of the constant velocity joint;
2) To prevent adverse effects of radial contraction of serrations and others caused by swaging without decreasing the axial effective length of the serrations and at low cost; and
3) To prevent rust in mating parts of the wheel hub and the serrated shaft to avoid deterioration of disassembling operation efficiency.